The invention relates to a force-sensing device for measuring a traction- and/or pressure force load in a structure, for example, in a bolt, hook, carrier, strut or the like. The force-sensing device comprises a force measuring sensor and a rod-shaped sensor carrier and is intended to be set into the loaded structure so that expansions and/or compressions of the structure under load are transferred to the force-sensing device and can be detected by its force measuring sensor.
It is desirable to provide an economical solution for force measurements in structures. In addition, a construction should be created that can be used to evaluate tractive forces, suspended loads, preferably for lifting devices or cranes.
In a preferred embodiment of an aspect of the invention the force-sensing device has a helical contour that is provided on the one end with a screw head that can be brought in engagement with a screwing tool. Even any other tool attack surface such as, for example, surfaces that contain a hole that can be brought in engagement with a pin as tool are conceivable as screw head. Therefore, it is also designated in the following as a force measuring dowel or an expansion measuring screw.
The force measuring dowel has a threaded section behind the head with which it can be screwed into a bore. On the other end the force measuring dowel has a tip via which the forces to be measured can be introduced into the force measuring dowel and has a middle or front section in which the force measuring sensor is fastened. The force measuring sensor consists of or comprises an element that can receive expansion and compression. The sensor is preferably constructed as a metallic thin-film element that is welded into the structure of the force measuring dowel.
At least four measuring resistors are integrated on the thin-film measuring element. Of them, two measuring resistors are applied for receiving expansion and two measuring resistors for receiving compression. The resistors are evaluated via a Wheatstone circuit and are run to the screw head via a cable lead-through to the other screw end and evaluated by an electronic component or by a display.
The welded-in force measuring sensor becomes a component of the structure of the expansion measuring screw and is elastically shaped or deformed when the expansion measuring screw is loaded or when the load is removed from it. The elastic deformations or shapings are introduced via the tip of the expansion measuring screw and conducted out via the threaded section. The sensor measuring resistors loaded at this time generate changes in resistance that are evaluated. As a result of the sensor construction, that is to be viewed as an atomic bond between the metallic sensor body and the layer to be measured, this construction has the particular advantage of being temperature-stable and zero-point-stable for a long time. A thin-film measuring sensor of high-grade steel, provided with a layer of glass is preferably used. Measuring resistors of a metallic type are again vaporized on and provided with contact pads and connected, e.g., by cable and evaluated.
Such a screw is brought, for example, into the structure of a load measuring hook. To this end the load measuring hook has a central bore provided on the upper end with a thread. The expansion measuring screw is inserted into this bore and tightened and pre-tensioned, during which preferably some air always remains at the upper screw head or the screw head therefore remains at a slight distance from the load measuring hook or its upper surface. This brings it about that when the screw is tightened in a defined manner and secured, it makes contact at the bottom of the bore with the load measuring hook and that the force measuring screw is subjected to a pre-tensioning. The middle and front section of the force measuring screw is placed under tension in such a manner by the forces applied by the screwing, tool via the upper threading on the screw in engagement with the load measuring hook that a certain pre-loading is adjusted at which the sensor element experiences a predefined pre-tensioning. In the following this achieved deformation is evaluated with a non-loaded load hook and defined as the zero point.
When the load measuring hook is loaded it is extended in length and the force measuring screw, that is under pre-tension, is partially relieved of the load.
If the round force measuring sensor consisting of or comprising high-grade steel was deformed slightly elliptically under pre-tensioning a change in the measuring resistors occurs during the loading of the loading hook by the relieving of the load. During the relieving of the load on the round expansion measuring sensor this sensor is returned back from the elliptical form into the previously round form.
This deformation or removal of the load from the measuring section can be associated in a linear manner with the load on the load measuring hook as a function of the load on the load measuring hook. The load measuring hook can also be constructed as a bolt that is held front the outside on an outer section, whereby its lower, free end has a projection with a positive receptacle for a suspension by force.
When four such bolts are used, they can also preferably be locked in container corners and be used in lifting devices for containers.
In this case, e.g., the measuring signal can be evaluated on all bolts at the four corners of the container and the total weight of the container determined.
Thus, however, it can also be determined when the container is lifted whether it is still locked at its lower section on a receptacle or is being taken up in a freely suspended manner.
Furthermore, in the case of a freely-suspended container locked in an orderly manner a conclusion can be made by the evaluation of the four bolts provided with this described measuring technology about the load distribution and the distribution of cargo in the container itself.
Upon the detection of an inadmissible load of a bolt relative to the others a clamping of the container during lifting can be assumed and a rapid turning off of the lifting crane can be initiated.